Self-calibration for color image reproduction system

ABSTRACT

A recalibration transformation is derived for a color image reproduction system having an input device and an output device by obtaining a medium conveying a selected color within a current gamut of the output device represented by first values in a first color space, scanning the medium with the input device to obtain second values representing the selected color in a second color space, applying a calibration transformation to map the second values to third values in the first color space, deriving an error transformation that maps from the first values to the third values, and deriving the recalibration transformation from an inverse of the error transformation that maps from the third values to the first values.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application entitled“Device-Independent and Medium-Independent Color Matching Between anInput Device and an Output Device” identified by attorney docket, Ser.No. 09/050,860 U.S. patent application entitled “Improved ScannerCalibration Technique to Overcome Tone Inversion” identified by attorneydocket, Ser. No. 09/050,866 and U.S. patent application entitled“Improved Color Matching Accuracy Inside and Outside the Gamut”identified by attorney docket, Ser. No. 09/050,862 all filedconcurrently with this application.

TECHNICAL FIELD

The present invention relates generally to color image reproductionsystems, and relates more particularly to features that improve colormatching between original color images and reproductions of thoseimages.

BACKGROUND ART Overview

Color image reproduction systems typically include an input device forobtaining a representation of an original image, an output device forgenerating a replica of the image, and a controlling device thatprocesses signals received from the input device to generate new signalssent to the output device to produce the replica, which preferably is ahigh-fidelity reproduction of the original image. The controlling devicemay be implemented by a general-purpose computer with appropriatesoftware and/or hardware for peripheral control and signal processing.Examples of an input device include hand held, flatbed and sheet-fedoptical scanners, digital and video cameras, and software applications.In other words, an original image may be sensed or it may be created bya process. Examples of an output device include ink jet, laser andphotolithography printers, electrostatic, flatbed and drum plotters, andvideo displays such as cathode ray tubes, thin-film-transistor andliquid crystal display panels.

Generally, input and output devices use some device dependentcolor-coordinate system to specify colors. These coordinate systems areoften specified in some device-dependent color space that convenientlymaps the color coordinates to the color-sensing or color-generatingprocess of the particular device. The term “color space” refers to anN-dimensional space in which each point corresponds to a particularcolor.

One example of a three-dimensional color space is an RGB space in whichpoint coordinates specify particular amounts of red (R), green (G) andblue (B) colorant that additively combine to represent a specific color.The operation of many scanners and color display devices may beconveniently controlled by signals that are specified in RGB space. Anexample of a four-dimensional color space is a CMYK color space in whichpoint coordinates specify particular amounts of cyan (C), magenta (M),yellow (Y) and black (K) colorant that subtractively combine torepresent a specific color. Another example is the three-dimensional CMYcolor space. The operation of many ink jet and laser printers may beconveniently controlled by signals that are specified in CMYK space orCMY space. Other color spaces that are related to particular devices arealso known.

Many practical devices are capable of sensing or reproducing only aportion of the full range of colors that can be discerned by a humanobserver. A device “gamut” refers to the range of colors that can besensed or reproduced by a particular device. For example, the gamut of aparticular scanner refers to the range of colors that can be sensed bythat scanner and the gamut of a particular printer refers to the rangeof colors that can be reproduced or printed by that printer.

A scanner gamut is determined by a variety of factors including thespectral response of the optical sensors, the spectral characteristicsof color filters, spectral characteristics of the illumination sourceand the resolution and linearity of analog-to-digital converters.

A printer gamut is determined by a variety of factors including spectralcharacteristics of colorants such as ink, spectral and porositycharacteristics of media such as paper, resolution or dots-per-inch ofthe printed image, half-toning methods and use of dithering, if any.

A video display gamut is determined by a variety of factors includingspectral characteristics of the light emitting material, type of displaydevice, resolution of pixels or video lines, and excitation voltage.

Although it is possible in principle to construct a color imagereproduction system by merely connecting an output device directly to aninput device, the results generally would not be satisfactory becausethe device-dependent coordinate systems and color spaces for the inputand output devices are generally not the same. Even if the two sets ofcoordinate systems and color spaces are the same, the fidelity of thereproduced image as compared to an original image would probably be verypoor because the gamut of the input device generally is not co-extensivewith the gamut of the output device. Values representing “out-of-gamut”colors that are not in the output device gamut cannot be reproducedexactly. Instead, some “in-gamut” color that is in the gamut of theoutput device must be substituted for each out-of-gamut color.

Color image reproduction systems can achieve high-fidelity reproductionsof original images by applying one or more transformations or mappingfunctions to convert point coordinates in one color space intoappropriate point coordinates in another color space. Thesetransformations may be conveniently performed by the controlling device,mentioned above. In particular, with respect to the output device gamut,transformations are used to convert values representing in-gamut andout-of-gamut colors in an input-device-dependent color space (DDCS) intovalues representing in-gamut colors in an output-DDCS. The mapping ofin-gamut colors and out-of-gamut colors is discussed separately.

Mapping In-Gamut Colors

The transformation of output device in-gamut colors for many practicaldevices are non-linear and cannot be easily expressed in some analyticalor closed form; therefore, practical considerations make accurateimplementations difficult to achieve. Many known methods implement thesetransformations as an interpolation of entries in a look-up table (LUT)derived by a process that essentially inverts relationships betweendevice responses to known input values. For example, a transformationfor an input device may be derived by using a medium conveying patchesof known color values in some device-independent color space (DICS) suchas the Commission International de L'Eclairage (CIE) 1931 XYZ space,scanning the medium with the input device to generate a set ofcorresponding values in some input-DDCS such as RGB color space, andconstructing an input LUT comprising table entries that associate theknown color XYZ values with the scanned RGB values. In subsequent scansof other images, scanned RGB values can be converted intodevice-independent XYZ values by finding entries in the input LUT havingRGB values that are close to the scanned values and then interpolatingbetween the associated XYZ values in those table entries. Variousinterpolation techniques such as trilinear, prism, pyramidal andtetrahedral interpolation may be used.

Similarly, a transformation for an output device may be derived byproducing a medium with color patches in response to color valuesselected from some output-DDCS such as CMYK color space, determining thecolor value of the patches in a DICS such as CIE XYZ space by measuringthe patches using a spectral photometer, and constructing an output LUTcomprising table entries that associate the measured color XYZ valueswith the corresponding CMYK values. In subsequent output operations, XYZcolor values can be converted into device-dependent CMYK values byfinding entries in the output LUT having XYZ values that are close tothe desired values and then interpolating between associated CMYK valuesin those table entries. Various interpolations such as those mentionedabove may be used.

In operation, a color image reproduction system scans an original imageto obtained scanned value in some input-DDCS, transforms the scannedvalues into some DICS, transforms these device-independent values fromthe DICS into some output DDCS and, in response, generates a replica ofthe original image. As mentioned above, the transformations describedthus far apply only to output device in-gamut colors.

Mapping Out-of-Gamut Colors

By definition, output device out-of-gamut colors cannot be reproducedexactly. Instead, high-quality color image reproduction systems usetransforms or mapping functions that substitute an in-gamut color foreach out-of-gamut color. Preferably, these transforms attempt tominimize the perceptible difference between each out-of-gamut color andthe corresponding substitute in-gamut color.

Techniques for transforming out-of-gamut colors into in-gamut colorsgenerally map the out-of-gamut colors to the boundary of the outputdevice gamut or compress a region of color space so that all desiredcolors are mapped into the output device gamut. U.S. Pat. No. 5,185,661describes a technique which seeks to preserve the hue of out-of-gamutcolors. The technique disclosed in U.S. Pat. No. 5,450,216 seeks tominimize perceptible changes in luminance or chrominance. U.S. Pat. No.5,491,568 discloses a technique that projects out-of-gamut colors ontothe gamut boundary along a line orthogonal to a gray line in colorspace. In U.S. Pat. No. 5,692,071, a disclosed technique maps eachout-of-gamut color to the closest entry in a LUT. The techniquedisclosed in U.S. Pat. No. 5,712,925 divides the output device gamutinto a higher-fidelity region and a lower-fidelity region and compressesall color space outside the higher-fidelity region into thelower-fidelity region.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a self-calibrationtechnique allowing a color image reproduction system to compensate forchanges in the output device gamut such as that caused by agingcomponents.

According to the teachings of one aspect of the present invention, arecalibration transformation is derived for a color image reproductionsystem comprising an input device and an output device by obtaining amedium conveying a selected color within a current gamut of the outputdevice represented by first values in a first color space, scanning themedium with the input device to obtain second values representing theselected color in a second color space, applying a calibrationtransformation to map the second values to third values in the firstcolor space, deriving an error transformation that maps from the firstvalues to the third values, and deriving the recalibrationtransformation from an inverse of the error transformation that mapsfrom the third values to the first values.

According to the teachings of another aspect of the present invention, arecalibration transformation is used in a color image reproductionsystem comprising an input device and an output device, wherein therecalibration transformation accounts for differences between anoriginal gamut of the output device and a current gamut of the outputdevice by scanning a first medium conveying an image having one or morein-gamut colors that are within the current gamut and one or moreout-of-gamut colors that are not within the current gamut to generaterespective first values representing the colors in a first color space,applying a calibration transformation to map the first values to secondvalues representing the colors in a second color space, applying therecalibration transformation to map second values representing only thein-gamut colors to third values, and generating a second mediumconveying a replica of the image, wherein the replica is generated inresponse to the third values and to second values representing onlyout-of-gamut colors.

The various features of the present invention and its preferredembodiments may be better understood by referring to the followingdiscussion and the accompanying drawings in which like referencenumerals refer to like elements in the several figures.

The contents of the following discussion and the drawings are set forthas examples only and should not be understood to represent limitationsupon the scope of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates major components in a typical color imagereproduction system.

FIGS. 2A and 2B illustrate major components for deriving transformationsfor input and output devices.

FIGS. 3A and 3B are schematic representations of points and regions incolor spaces, particularly corresponding points and regions in CIE XYZspace and CIE L*a*b* space.

FIG. 4 is a schematic representation of two hypothetical gamuts within atwo-dimensional projection of a normalized color space.

FIGS. 5A and 5B are schematic representations of points and hypotheticalgamut boundaries in CIE L*a*b* space.

FIGS. 6A and 6B are schematic representations of points andtransformational mappings onto the boundary of a hypothetical gamut inCIE L*a*b* space.

FIG. 7 is a schematic representation of respective gamuts for twoprinters or, alternatively, one printer operating in two differentmodes.

FIG. 8 is a schematic representation of a shift in the gamut of ahypothetical printer such as that caused by component aging, forexample.

FIG. 9 illustrates major components in one embodiment of a controllingdevice that achieves an improved degree of device and mediaindependence.

FIG. 10 illustrates major components in a color image reproductionsystem that provides recalibration of selected colors for an outputdevice.

FIG. 11 illustrates one embodiment of a recalibration transformation.

FIG. 12 illustrates major components in a typical personal computer thatmay be used to implement various aspects of the present invention.

FIGS. 13A through 13C illustrate the end-to-end effects of input oroutput devices and the system components that implement devicetransformations.

FIG. 14 is flowchart illustrating one method for deriving atransformation to overcome tone inversion.

FIG. 15 illustrates major components for deriving a transformation toovercome tone inversion.

FIG. 16 is flowchart illustrating one method for improving the accuracyof a transformation for in-gamut colors.

FIG. 17 is flowchart illustrating one method for improving the accuracyof a transformation for out-of-gamut colors.

FIG. 18 is flowchart illustrating one method for deriving arecalibration transformation

FIG. 19 is a flowchart illustrating one method for using a recalibrationtransformation.

MODES FOR CARRYING OUT THE INVENTION Color Image Reproduction SystemOverview

FIG. 1 illustrates major components in a typical color imagereproduction system. Input device 10 receives from path 11 signalsrepresenting an original image and generates along path 12 aninput-device-dependent representation of the original image. Controllingdevice 20 receives this representation from path 12 and, in response,generates along path 31 an output-device-dependent representation of theoriginal image. Output device 30 receives this representation from path31 and, in response, generates along path 32 a replica of the originalimage. The present invention is directed toward improving the fidelitywith which the replica reproduces the original image.

Input device 10 may be essentially any type of scanner, camera ordigital graphics application. If input device 10 is an optical scanner,for example, the signals received from path 11 could be considered to beoptical. If input device 10 is an application for creating ormanipulating color images, for example, the signals received from path11 could be considered to represent commands or data. Throughout thisdisclosure, more particular mention will be made of optical scanners;however, many of the principles and features of the present inventionmay be applied in systems incorporating other types of input devices.

Output device 30 may be essentially any type of printer, plotter ordisplay. If output device 30 is an ink-jet printer, for example, thereplica generated along path 32 could be considered to be the printedimage. If output device 30 is a CRT or TFT display, for example, thereplica generated along path 32 could be considered to represent theimage formed on the display device. Throughout this disclosure, moreparticular mention will be made of printers; however, many of theprinciples and features of the present invention may be applied insystems incorporating other types of output devices.

By its very nature, the characteristics of the input-device-dependentrepresentation generated along path 12 depends on the characteristics ofinput device 10. Many optical scanners, for example, generate signalsrepresenting colors as points with red (R), green (G) and blue (B)coordinates in an RGB device-dependent color space (DDCS). For ease ofdiscussion herein, the input-DDCS will generally be referred to as RGBspace; however, many other color spaces and representations may be usedto practice the present invention.

Similarly, the characteristics of the output-device-dependentrepresentation generated along path 31 are chosen to match thecharacteristics of output device 30. Many color printers, for example,generate images in response to values representing cyan (C), magenta(M), yellow (Y) and black (K) coordinates in a CMYK DDCS. Many displaydevices such as cathode ray tubes or thin-film-transistor panelsgenerate images in response to values representing red, green and bluein an RGB DDCS. Because of the device-dependent nature of these colorspaces, scanner RGB spaces should not be equated to display RGB spaces.For ease of discussion herein, the output-DDCS will generally bereferred to as CMYK space; however, many other color spaces andrepresentations may be used to practice the present invention.

Controlling device 20 is responsible for transforming signalsrepresenting the original image in the input-DDCS into signalsrepresenting the same image in the output-DDCS. This may be accomplishedby using input-device map 21 to transform the input-DDCS signals into arepresentation in a device-independent color space (DICS), and usingoutput-device map 23 to transform the DICS representation into thesignals representing the same image in the output-DDCS. Controllingdevice 20 may include other transformations and processes such as thosedescribed herein.

Controlling device 20 may be implemented by software and/or hardware ina general-purpose computer such as that illustrated in FIG. 12. FIG. 12is a functional block diagram of one embodiment of a typical personalcomputer system 40.CPU 42 provides computing resources. I/O control 43represents an interface to I/O device 44 such as a keyboard, mouse ormodem. RAM 45 is system random access memory. Storage control 46represents an interface to storage device 47 that includes a storagemedium such as magnetic tape or disk, or an optical medium. The storagemedium may be used to record programs of instructions for operatingsystems, utilities and applications, and may include embodiments ofprograms that implement various aspects of the present invention.Display control 48 provides an interface to display device 49. Control50 represents an interface to scanner 51 which is an input device likean optical scanner. Control 52 represents an interface to printer 53which is an output device like an ink jet color printer. Devices likescanner 51 may serve as input device 10 and devices like display device49 or printer 53 may serve as output device 30.

In the embodiment shown, all major system components connect to bus 41which may represent more than one physical bus. For example, somepersonal computers incorporate only a so called Industry StandardArchitecture (ISA) bus. Other computers incorporate an ISA bus as wellas a higher bandwidth bus conforming to some bus standard such as theVESA local bus standard or the PCI local bus standard. Preferably,display control 48 connects to a high-bandwidth bus to improve the speedof display. A bus architecture is not required to practice the presentinvention.

The functions of one or more computer components as well as variousaspects of the present invention can be implemented in a wide variety ofways including discrete logic components, one or more ASICs and/orprogram-controlled processors.

Controlling device 20 may also be implemented by a special-purposedevice. The manner in which controlling device 20 is implemented is notimportant to the present invention. For example, the followingdisclosure will sometimes refer to implementations that store tables inRAM merely for ease of discussion. Other implementations includingdigital and analog processing circuitry may used.

Derivation of Input and Output Maps

FIGS. 2A and 2B illustrate major components for deriving input-devicemap 21 and output-device map 23. These illustrations and the followingdiscussion are presented merely as examples to illustrate principles.These maps or transformations may be derived in other ways.

Referring to FIG. 2A, input-device map 21 may be derived by scanning animage 15 that has known color characteristics. For example, image 15 maybe one or more sheets of paper with areas or “patches” of known color.The color characteristics of these patches may be determined by measuredevice 60 such as a spectral photometer or calorimeter. According to thetechnique shown in the figure, measure device 60 scans image 15 andgenerates signals along path 61 representing the colors of the patchesin some DICS such as the Commission International de L'Eclairage (CIE)1931 XYZ space, referred to herein as CIE XYZ space. Input device 10scans image 15 and generates signals along path 12 representing thecolors of the patches in an input-DDCS such as scanner RGB space.

The device-independent and the device-dependent representationsgenerated along paths 61 and 12, respectively, provide selected pointsin the two color spaces that define a forward function ƒ_(I)representing the way in which input device 10 converts real-world colorsinto a device-dependent representation. In response to these signals,calibration device 65 derives input-device map 21 which is an inversefunction ƒ_(I) ⁻¹ from the DCCS to the DICS. For example, if measuredevice 60 generates values in CIE XYZ space and input device 10generates signals in some RGB space, then the forward functioncorresponding to input device 10 may be denoted as ƒ_(I): XYZ→RGB andthe inverse function corresponding to input-device map 21 may be denotedas ƒ_(I) ⁻¹: RGB→XYZ.

The way in which these two components work together is illustrated inFIG. 13A. Input device 10 effects a transformation ƒ_(I) on valuesrepresenting real-world colors to obtain values in some input-DDCS. Itis often convenient to represent real-world colors in some DICS such asCIE XYZ space. The transformation may then be expressed as a mappingfrom CIE XYZ space to some input-DDCS such as an RGB space as describedabove. Input-device map 21 effects a transformation ƒ_(I) ⁻¹on thevalues in the input-DDCS to obtain mapped values in some DICS such asCIE XYZ space. The end-to-end effects of these two components is toeffect a transformation from one DICS to another DICS. According to theexample discussed above and illustrated in the figure, thetransformation is, in principle, similar to the identify matrix thatmaps from CIE XYZ space to CIE XYZ space, which may be denoted as F_(I):XYZ→XYZ. In practice, however, arithmetic round off errors andinterpolation errors introduce noise into the process.

Referring to FIG. 2B, output-device map 23 may be derived by usingoutput device 30 to generate image 35 and determining the colorcharacteristics of image 35. For example, image 35 may be one or moresheets of paper with patches that are analyzed by measure device 62 suchas a spectral photometer or colorimeter. According to the techniqueshown in the figure, measure device 62 scans image 35 and generatessignals along path 63 representing the colors of the patches in someDICS such as CIE XYZ or CIE L*a*b* space. Output device 30 or somecomponent controlling output device 30 generates signals along path 33representing the patches in some output-DDCS such as printer CMYK space.

The device-independent and the device-dependent representationsgenerated along paths 63 and 33, respectively, provide selected pointsin the two color spaces that define a forward function ƒ_(o)representing the way in which output device 30 converts thedevice-dependent representation into real-world colors. In response tothese signals, calibration device 67 derives output-device map 23 whichis an inverse function ƒ_(o) ⁻¹from the DICS to the DDCS. For example,if measure device 62 generates values in CIE L*a*b* space and outputdevice 30 generates the image in response to signals in some CMYK space,then the forward function corresponding to output device 30 may bedenoted as ƒ_(o): CMYK→L*a*b* and the inverse function corresponding tooutput-device map 23 may be denoted as ƒ_(o) ⁻¹: L*a*b*→CMYK.

The way in which these two components work together is illustrated inFIG. 13B. Output-device map 23 effects a transformation ƒ_(I) ⁻¹onvalues representing colors in some DICS to obtain values in someoutput-DDCS. Output device 30 effects a transformation ƒ_(I)on thevalues in the output-DDCS to obtain a replica image with real-worldcolors. If the real-world colors are expressed in some DICS such as CIEXYZ space, the transformation may then be expressed as a mapping fromthe output-DDCS to CIE XYZ as described above. The end-to-end effects ofthese two components is to effect a transformation from one DICS toanother DICS. According to the example discussed above and illustratedin the figure, the transformation maps from CIE L*a*b* space to CIE XYZspace, which may be denoted as F_(o): L*a*b*→XYZ.

FIGS. 3A and 3B are schematic representations of points and regions intwo color spaces. These figures illustrate corresponding points andregions in CIE XYZ space and CIE L*a*b* space, discussed more fullybelow; however, these figures are useful to illustrate principles of amapping relationship between arbitrary color spaces. As shown in thefigures, points 101-104 in one color space correspond to points 111-114,respectively, in another color space. The points along the fourstraight-line segments connecting these references points in the colorspace of FIG. 3A space correspond to points along the curved andstraight-line segments connecting the referenced points in the colorspace of FIG. 3B.

As these figures show, the correspondence is often non-linear. Becausethe transformation between color spaces usually cannot be expressed in aclosed or analytical form, these transformations are often implementedby a look-up table, from which values of intermediate points may beobtained by interpolation.

For reasons that are discussed below, preferred embodiments of systemsincorporating scanners and printers use two DICS. Scanner signals aremapped into CIE XYZ space and printer signals arc mapped from CIE L*a*b*space. It is, therefore, necessary to provide a map or transformationfrom CIE XYZ to CIE L*a*b* space. This transformation may be denotedƒ_(T): XYZ→L*a*b*. As mentioned above, this transformation isillustrated in FIGS. 3A and 3B. In such embodiments, controlling device20 converts signals received from path 12 into signals generated alongpath 31 according to a transformation ƒ_(c) that is equivalent to acascaded application of the transformations discussed above, denotedhere aspic ƒ_(c)=ƒ_(I) ⁻¹•ƒ_(T)•ƒ_(o) ⁻¹, or ƒ_(c): RGB→CMYK=ƒ_(I) ⁻¹:RGB→XYZ•ƒ_(T): XYZ→L*a*b*•ƒ_(o) ⁻¹: L*a*b*→CMYK.

The effect of this transformation in conjunction with the othertransformations is illustrated in FIG. 13C. As explained above, inputdevice 10 and input-device map 21 effect a transformation from one DICSto another DICS such as from CIE XYZ space to CIE XYZ space, denoted asF_(I): XYZ→XYZ. Output-device map 23 and output device 30 effect atransformation from one DICS to another DICS such as from CIE L*a*b*space to CIE XYZ space, denoted as F_(o): L*a*b*→XYZ. By effecting atransformation from CIE XYZ space to CIE L*a*b* space, the ƒ_(T)transformation provides the link required to couple the F_(I) and theF_(o) transformations together.

The end-to-end effect of these coupled transformations represents theoverall operation of the color image reproduction system. According tothe example discussed above and illustrated in the figure, thisend-to-end effect is a mapping F_(s) from CIE XYZ space to CIE XYZ spacewhich is, as mentioned above, equivalent in principle to an identitymatrix. In absence of arithmetic round off errors and accuracy errors inthe component transformations, the color image reproduction system is atransparent system that is able to reproduce an original imageperfectly.

Unfortunately, even if the transformations could be implementedperfectly, reproduction errors still occur because practical input andoutput devices have limited gamuts that are generally not coextensive.As a result, the perceived accuracy of the replica depends on theability of the system to substitute an in-gamut color that isindistinguishable from each out-of-gamut color. This process issometimes referred to as gamut mapping.

Gamut Mapping

As mentioned above, input and output devices are capable of sensing orreproducing only a portion of the full range of colors that can bediscerned by a human observer. The “gamut” of a scanner, for example,refers to the range of colors that can be sensed by that scanner. Thegamut of a printer refers to the colors that can be generated by thatprinter. The colors that can be reproduced are referred to as “in gamut”colors and the colors that cannot be reproduced are referred to as“out-of-gamut” colors.

FIG. 4 is a schematic representation of two hypothetical device gamuts.Closed contour 120 represents a two-dimensional projection of anormalized color space, such as the CIE xy chromaticity diagram, thatrepresents the chromaticity of the visible spectrum. Colors are plottedin the diagram according to wavelength. The shortest wavelengths appearin the lower-left region within contour 120 and the longest wavelengthsappear in the lower-right region within contour 120.

The triangle with vertices at points 121-123 represents a device gamutthat is typically represented in an RGB space; however, as shown, theboundary of this gamut is plotted in CIE xy space. Vertex 121 representsthe extent to which the gamut includes colors in the red portion of thespectrum. The vertices at points 122 and 123 represent the extent towhich this gamut includes colors in the green and blue portions of thespectrum, respectively. Ignoring granularity due to the discrete natureof digital devices, a device having this gamut is capable of reproducingall of the colors inside this triangle.

Similarly, the polygon with three of its vertices at points 126-128represents a device gamut that is typically represented in CMY space;however, as shown, the boundary of this gamut is plotted in CIE xyspace. The vertices at points 126-128 correspond to colors in the cyan,magenta and yellow portions of the spectrum, respectively. Ignoringgranularity due to digital implementations, a device having this gamutis capable of reproducing all colors included within the polygon.

FIGS. 5A and 5B are schematic representations of points and hypotheticalgamut boundaries in CIE L*a*b* space. The L* coordinate representsluminance or brightness and the a*,b* coordinates represent color.Points having the same L* coordinate have the same luminance and pointshaving the same angle with respect to the a*,b* axes have the same coloror hue. The distance between a point and the L* axis is a measure ofchroma magnitude or chromaticity. Points along the L* axis representshades of gray from black to white which are neutral in color.

FIG. 5A illustrates two hypothetical gamuts 131 and 133 in L*a*b* colorspace as viewed along the L* axis. FIG. 5B illustrates gamuts 131 and133 as viewed along the b* axis. The gamut boundaries shown in FIG. 5Aas well as in other figures are intended to provide simple illustrationssuitable for understanding principles of the present invention. They arenot intended to represent the boundaries of gamuts for actual deviceswhich are generally more complex.

Circles 131 and 133 shown in FIG. 5A illustrate the locus of points inplane 130 at the boundaries of the two gamuts. Point 134 is inside gamut131 but outside gamut 133. Point 136 is inside both gamuts. As may beseen from the two figures, gamut 131 includes a greater range of colorsand luminance than does gamut 133. The gamut of a typical scanner oftenincludes a greater range of colors than does the gamut of a typicalprinter. It is not uncommon, however, for a printer gamut to includesome colors that are outside a scanner gamut. This situation poses nodifficulty because the scanner inherently maps such colors into its owngamut.

Mapping In-Gamut Colors

As mentioned above, transformations from a first color space to a secondcolor space are often non-linear and usually difficult if not impossibleto express in some closed or analytical form. These transformations aregenerally implemented by an approximation technique such asmulti-dimensional interpolation of entries in a LUT. Each entry in theLUT contains coordinates of a specific point in the first color spaceand coordinates of the corresponding point in the second color space.For any arbitrary point in the first color space, the coordinates of thecorresponding point in the second color space can be approximated byinterpolating coordinates of selected table entries. Trilinear, prism,pyramidal and tetrahedral interpolation techniques and a numbervariations of such techniques are known; however, some form oftetrahedral interpolation is generally preferred.

According to tetrahedral interpolation, the LUT is searched to findentries representing points in the first color space that define thevertices of the smallest tetrahedron that encloses the arbitrary point.Interpolation coefficients are calculated based on the relative positionof the arbitrary point with respect to the four vertices. Finally, anapproximation of the mapped point is obtained by using the interpolationcoefficients to form a linear combination of the coordinates in thesecond color space that correspond to the four vertices. This linearcombination may be represented as:

x _(P) =a ₁ x ₁ +a ₂ x ₂ +a ₃ x ₃ +a ₄ x ₄

y _(P) =a ₁ y ₁ +a ₂ y ₂ +a ₃ y ₃ +a ₄ y ₄

z _(P) =a ₁ z ₁ +a ₂ z ₂ +a ₃ z ₃ +a ₄ z ₄

where x_(P)=the point in second color space corresponding to thearbitrary point,

a₁ through a₄ are the coefficients of interpolation,

(x₁,y₁,z₁,)=coordinates of vertex 1 in the second color space,

(x₂,y₂,z₂)=coordinates of vertex 2 in the second color space,.

(x₃,y₃,z₃)=coordinates of vertex 3 in the second color space, and

(x₄,y₄,z₄)=coordinates of vertex 4 in the second color space.

Additional information regarding various forms of interpolation may beobtained from H. R. Kang, “Color Technology for Electronic ImagingDevices,” SPIE Optical Engineering Press, 1997, pp. 64-101, 141-152 and248-251, which is incorporated herein by reference.

Mapping Out-of-Gamut Colors to Gamut Boundary

In preferred embodiments, out-of-gamut colors are mapped onto theboundary of the gamut by projecting the color onto the line in colorspace representing neutral (gray) colors and clipping the projection atthe gamut boundary. This can be performed conveniently in CIE L*a*b*space where points on the L* axis represent neutral colors.

Referring to FIG. 6A, the out-of-gamut color represented by point 134 ismapped to the color represented by point 135 on the gamut boundary byprojecting a line from point 134 to the L* axis and finding theintersection of the projection with the boundary. If possible, theprojection is orthogonal to the L* axis so that the luminance level ispreserved.

An orthogonal projection to the L* axis is not always possible.Referring to FIG. 6B, the colors represented by points 141 and 144 haveluminance levels that lie outside the gamut. The projection from point141 is made to point 142, which represents the minimum value for L* inthe gamut along the neutral-color line. The projection from point 144 ismade to point 145, which represents the maximum value for L* in thegamut along the neutral-color line.

This mapping may be performed in other color spaces. For example, in CIEXYZ space, the points of neutral color define a line for whichx/x₀=y/y₀=z/z₀, where (x₀,y₀,z₀) specifies normalization coordinates forthe color space. An out-of-gamut color may be mapped to the intersectionof the gamut boundary with a curve connecting the point to this line.The curve corresponds to the orthogonal projection in L*a*b* space thatpreserves luminance. Alternatively, a straight-line projection or someother curve can be used in XYZ space, which will provide a differentresult.

Extending the Color Range for the Input-Device Gamut

The accuracy of input-device map 21 provided by a technique such as thatillustrated in FIG. 2A is enhanced if the range of colors conveyed byimage 15 is at least as broad as the gamut of input device 10. In otherwords, it is very likely that the accuracy of input-device map 21 willbe very poor in those regions of color space that are not represented byany patches in image 15. This poor accuracy can cause an input-devicetransformation to map colors in a way that does not preserve relativechroma magnitudes.

Referring to FIG. 7, point 165 represents an out-of-gamut color that hasa chroma magnitude greater than the chroma magnitude of an in-gamutcolor represented by point 155. If input-device map 21 is derived fromonly the colors available from gamut 150, then a color represented by apoint such as point 165 will be mapped by interpolation to point 167inside the boundary of gamut 150. The color will be mapped inside thegamut boundary because the four points defining the smallest tetrahedronthat encloses point 165 are all on the gamut boundary. In other words,because input-device map 21 is formed from only colors found withingamut 150,there are no points outside the gamut to provide a vertex forinterpolation. The extent to which a point representing an out-of-gamutcolor is mapped to a point inside the boundary will depend on theaccuracy and resolution of the interpolation points and the convexity ofthe local gamut boundary. If the accuracy of the input-devicetransformation is sufficiently poor or the convexity of the local gamutboundary is sufficiently large, the chroma magnitude of the colorrepresented by point 167 may be less than the chroma magnitude of thecolor represented by point 155. This result may be perceived as aninversion of chromaticity, sometimes referred to as tone inversion.

Tone inversion may be avoided or at least reduced by using a broaderrange of colors to derive the input-device map. One way to produce abroader range of colors is to use a printer or other output device witha very broad gamut to generate image 15. Such devices are usuallyexpensive or difficult to acquire.

Another way to produce a broader range of colors is to use multipleprinters that have widely varying gamuts or to operate one or moreprinters in multiple modes. In this context, “mode” refers to a numberof operating conditions including printing resolution (dpi), choice ofcolorants or media, and half-toning methods or stochastic screeningtechniques.

FIG. 7 is a schematic representation of respective gamuts 150 and 160for two printers or, alternatively, one printer operating in twodifferent modes. For ease of explanation, either or both of thesesituations are included in the following discussion that refers togamuts of different printers.

FIG. 14 illustrates one method for deriving an input-device map 21 toovercome tone inversion. First, using a technique similar to thatdescribed above, an inverse function g_(I) ⁻¹ is obtained (step S241)for the points in gamut 150 representing colors printed by the firstprinter. Similarly, an inverse function g₂ ⁻¹ is obtained (step S242)for the points in gamut 160 representing colors printed by the secondprinter. If a color is represented by points in both gamuts (step S243)such as those represented by points 152 and 162,the inverse functionƒ_(I) ⁻¹ for input-device map 21 is equal to the average of the inversefunctions (step S244). The “same” color from two sets of patches willnot necessarily be mapped by the input device to the same point in DDCS.The difference in mapped points may occur due to noise in the scanningprocess and/or because of minor differences in the actual colors.

If a color is represented by a point in only gamut 150 (step S245) suchas that represented by point 151,the inverse function ƒ_(I) ⁻¹ is equalto the inverse function g_(I) ⁻¹ obtained for that gamut (step S246). Ifa color is represented by a point in only gamut 160 (step S247) such asthat represented by point 166, the inverse function ƒ_(I) ⁻¹ is equal tothe inverse function g_(I) ⁻¹ obtained for that gamut (step S248). If acolor is represented by point that is in neither gamut such as the colorrepresented by point 165, a projection is made to a line representingneutral colors (step S249) and the points p₁ and p₂ of intersection ofthat projection with the boundary of gamut 150 and 160, respectively, isdetermined (step S250). If the point of intersection p₁ with gamut 150represents a color with a greater chroma magnitude than the point ofintersection p₂ with gamut 160 (step S251), then point p₁ is selected tobe the mapped point (step S252). Otherwise, point p₂ is selected to bethe mapped point (step S253). Alternatively, the point of intersectionrepresenting the color with the largest magnitude chroma coordinate maybe selected.

In the example shown in FIG. 7, the projection from point 165 intersectsthe boundaries of the two gamuts at points 166 (point p₂) and 167 (pointp₁). The color represented by point 166 has the larger chroma magnitude;therefore, point 166 is selected to define the mapping for the colorrepresented by point 165.

This relationship may be summarized as follows:

ƒ_(I) ⁻¹(p)=½[g ₁ ⁻¹(p)+g ₂ ⁻¹(p)]

if p is in both gamut 1 and gamut 2;

ƒ_(I) ⁻¹(p)=g ₁ ⁻¹(p)

if p is in gamut 1 but not in gamut 2;

ƒ_(I) ⁻¹(p)=g ₂ ⁻¹(p)

if p is in gamut 2 but not in gamut 1;

ƒ_(I) ⁻¹(p)=g ₂ ⁻¹(p ₁)

if p is in neither gamut 1 nor gamut 2 and the chroma of p₁ is greaterthan the chroma of p₂; and

ƒ_(I) ⁻¹(p)=g ₂ ⁻¹(p ₂)

otherwise,

where p₁=the intersection of a projection of point p onto theneutral-color line and the boundary of gamut 1, and

p₂=the intersection of a projection of point p onto the neutral-colorline and the boundary of gamut 2.

FIG. 15 illustrates major components of one way for deriving atransformation in this manner. Check gamuts 82 receives information frompath 81 representing one or more points in color space and determineswhether a respective point p is in either or both of two gamuts. Ifpoint p is in the first gamut, information representing that point ispassed along path 83 to transformation 91 which implements inversefunction g_(I) ⁻¹ discussed above. If point p is in the second gamut,information representing that point is passed along path 84 totransformation 92 which implements inverse function g₂ ⁻¹ discussedabove. If point p is in both gamuts, information is passed to bothtransformations and an indication of this is passed along path 85. Ifpoint p is in neither gamut, information representing that point ispassed along path 86.

Projection/clip chroma 87 projects a line from point p to theneutral-color line, determines the intersection or clip point p₁ of thisprojection with the boundary of the first gamut, and determines thechroma for point p₁. In one embodiment, the chroma is determined fromthe chroma magnitude which, in L*a*b* space, is equal to the square rootof the sum of the squares of the a* and b* coordinates. In anotherembodiment, the chroma is determined from the magnitude of the largestcoordinate which, in L*a*b* space is the larger of |a*| and |b*|.Similarly, projection/clip chroma 88 projects a line from point p to theneutral-color line, determines the intersection or clip point p₂ of thisprojection with the boundary of the second gamut, and determines thechroma for point p₂.

Compare 89 determines whether point p₁ or point p₂ has the largerchroma. If point p₁ has the larger chroma, compare 89 passes informationrepresenting point p₁ along path 83 to transformation 91. If point p₂has the larger chroma, compare 89 passes information representing pointp₂ along path 84 to transformation 92.

Select 93 receives the results of transformation 91 and transformation92 and, if the indication received from path 85 indicates point p is inboth gamuts, generates along path 94 information representing theaverage of the results received from both transformations. If theindication received from path 85 does not indicate point p is in bothgamuts, select 93 generates along path 94 information representing thesum of the results received from both transformations. In thisembodiment, it is assumed that if no information is passed to arespective transformation, that transformation produces a result equalto zero; therefore, the sum of the results will be equal to whichevertransformation was passed information.

This technique may be used with any number (N>1) of gamuts by obtainingan inverse transformation for each gamut, determining whether a point pis in all of the gamuts and, if so, taking the average of the respectiveinverse transformations for each gamut. If the point is not in allgamuts, a check is made to determine if point p is in some combinationof N-1 gamuts. If so, an average of the respective inversetransformations for those N-1 gamuts is taken. If not, a check is madefor all combinations of N-2 gamuts. This process continues until it isdetermined that point p is in none of the gamuts. A projection to theneutral-color line is made and a check is made to determine allintersections with various gamut boundaries. The intersection with thelargest chroma magnitude, or alternatively the largest chromacoordinate, is selected and the appropriate inverse transformation istaken of that point.

Device and Media Independence

In preferred embodiments, input-device map 21 is independent of outputdevice 30, output device map 23 is independent of input device 10, andboth maps are independent of the media used by output device 30 togenerate the replica. Nevertheless, if a color image reproduction systemuses multiple input devices, output devices and/or media types,considerable memory is required to store the transformation LUT foreach. The technique described below can reduce the amount of memoryrequired to store LUT by eliminating the need for multipletransformations to accommodate media variations and by simplifying theprocess required to accommodate differences in the dynamic range ofluminance for various device gamuts. One embodiment of controllingdevice 20 that comprises components providing these features isillustrated in FIG. 9.

A degree of media independence may be achieved by normalize 24, whichaccounts for differences in the “white point” for respective media. Thismay be accomplished conveniently in the ƒ_(T) transformation discussedabove that converts values from CIE XYZ space into CIE L*a*b* space. Apoint in XYZ space may be mapped into L*a*b* space by the followingnon-linear equations:

L*=116(y/y _(o))−16

a*=500[m(x/x _(o))−m(y/y _(o))]

b*=500[m(y/y _(o))−m(z/z _(o))]

where x,y,z=coordinates in CIE XYZ space,

x_(o),y_(o),z_(o)=maximum value for the coordinates in CIE XYZ space,

m(t)=t ^(⅓)

if 0.008856<t≦1, and

m(t)=7.787t+({fraction (16/116)})

if 0≦t≦0.008856 for the independent variable t.

By varying the values of the x_(o),y_(o),z_(o) coordinates, normalize 24can normalize the XYZ space according to a desired medium “white point”as the mapping into L*a*b* space is performed. As a result, variationsin media white point can be accommodated by an implementation thatrequires only enough memory to store one transformation LUT.

The desired white point for a particular medium can be determined byscanning the medium and transforming the scanned values using anappropriate transformation such as input-device map 21; however, moreaccurate results may be obtained if the white point is determined byanalyzing the medium with a spectral photometer.

Normalize 24 can be made responsive to a variety of input. For example,normalize 24 may select normalization parameters from a table inresponse to a signal received from path 13 such as that provided by auser activated switch or selected software option, or it may adaptnormalization parameters in response to a white point measurementprovided by input device 10 and input-device map 21. The measurement maybe taken during a special white-point calibration scan or possibly froman assessment of the predominant color in a scanned original image.

A greater degree of device independence may be achieved by compress 25,which accounts for differences in the range of luminance levels forinput- and output-device gamuts. Referring to FIG. 5B, suppose gamut 131represents a scanner gamut with a detectable range of luminance levelsfrom L*=0 to 100, and gamut 133 represents a printer gamut with areproducible range of luminance levels from L*=30 to 100. In the exampleshown, the two devices have the same upper limit but have differentlower limits. The technique discussed here may be applied to deviceswith respective luminance ranges that differ in essentially any way.

As explained above, out-of-gamut colors such as the color represented bypoint 141 in FIG. 6B may be mapped to the minimum luminance level for aneutral color in the printer gamut. Alternatively, such colors may bemapped into the printer gamut by compressing the luminance level. In theexample mentioned above, this could be accomplished by linearcompression of luminance levels in the range from L*=0 to 40 into therange from L*=30 to 40. Luminance levels from LX=40 to 100 areunchanged. Essentially any form of compression may be used; however,linear compression of this type is inexpensive to perform, provides somegradation in luminance level rather than merely clipping all low-levelcolors to the minimum, restricts changes in luminance to lower levelswhere changes are not easily perceived, and preserves luminance valuesat higher levels where changes are more easily perceived.

Luminance compression can be provided easily without requiring anelaborate transformation. As a result, luminance dynamic rangevariations in device-dependent gamuts can be accommodated and a degreeof device independent can be achieved without requiring significantadditional memory to store a LUT.

This compression can be made responsive to a variety of input. Compress25 may select compression parameters from a table in response to asignal received from path 14 such as that provided by a user activatedswitch or selected software option, or it may adapt the compressionparameters in response to signals identifying an output device that arereceived from the output device itself.

Preferably, after luminance compression, the resulting L*a*b*coordinates for each picture element (pixel) is compared to thresholdsand, if the color space coordinates representing that pixel have aprescribed relationship with respect to those thresholds, the L*a*b*coordinates are set to some specified values, say (100,0,0). This may berepresented by the following pseudo code fragment:

if L*>L_(TH) and |a*|<a_(TH) and |b*|<b_(TH) then

set L* equal to 100

set a* equal to 0

set b* equal to 0

The thresholds may be specified as L_(TH)=95, a_(TH)=2 and b_(TH)=3, forexample.

This process is represented by white point background 26 in FIG. 9. Thespecified values represent a desired white point for the output replica.This process tends to remove artifacts from the replica that are createdby noise in the scanning process and by arithmetic round off errors inthe transformation processes. This operation may also be used to removethe background color of the original medium without changing the colorsof the reproduced image.

Improving Accuracy of an Input Device Map

As mentioned above, transformations are often implemented by aninterpolation of entries in a look-up table (LUT). Because thetransformations are non-linear, the accuracy of the interpolation isaffected by the distance between points represented by adjacent entriesin the table.

As discussed above in connection with FIGS. 3A and 3B, a point canmapped from CIE XYZ space into CIE L*a*b* space by a set of analyticalexpressions. This type of transformation is not available for many colorspace mappings but it is useful to illustrate the accuracy ofinterpolation.

Point 107 is shown in FIG. 3A to be within a tetrahedron that hasvertices at points 101-104. By applying tetrahedral interpolation tocorresponding points 111-114 in L*a*b* space according to the relativeposition of point 107 with respect to the tetrahedral vertices, anapproximate position can be obtained for the corresponding point inL*a*b* space. This approximate location is illustrated as point 118. Byapplying the three analytical expressions discussed above, an exactmapping in L*a*b* space for point 107 can be obtained. This exactmapping is shown as point 117. The distance between points 117 and 118represents the interpolation error.

One common method for improving the accuracy of interpolation is toincrease the density of the points represented in the LUT. Although thissolution is simple in concept, it is often impractical because of theincrease in cost for the memory required to store the LUT.

The interpolation accuracy of an input-device map may be improvedwithout increasing the number of LUT entries by modifying some of thepoints represented in the LUT for selected regions of color space. Oneimportant region of color space that generally requires high accuracy isthe region containing the points of neutral color. In some color spacessuch as CIE L*a*b*, these points are coincident with the L* axis, ora*=be=0.

The accuracy of interpolation for a selected region of a mapped colorspace can be improved by obtaining color patches for a selected colorwithin the selected region, measuring the patch as necessary to obtainappropriate values in a DICS, scanning the patch to obtain appropriatevalues in the input-DDCS, and modifying entries in the LUT representingpoints adjacent to the selected color. Patches of selected colors may beobtained by choosing points that are interior to each region ofinterest, mapping the points to the appropriate output-DDCS, generatingthe appropriate patches. If the selected color is outside the gamut of aparticular output device, another output device may be used.

The table entries corresponding to the four vertices are modifiedaccording to two different processes depending upon whether the regionof interest is inside or outside the gamut. The process of improvingaccuracy for regions inside the gamut will be discussed first.

Improving Accuracy of In-Gamut Colors

In a preferred embodiment, the entries in the LUT correspond touniformly spaced grid points in a color space. Because it is difficultif not impossible to obtain color patches for specific grid points inthe color space, the values for essentially all of the LUT entries arecalculated by interpolation of the values that can be obtained fromactual color patches. The perceptible effect caused by the errorsintroduced by this process depend on a number of factors including therelative spacing of the color patches in the color space and the varyingsensitivity of the human observer to changes in color across the colorspace.

The accuracy of a LUT derived in this manner can be improved byobtaining a very large number of patches having colors represented bypoints closely spaced in the color space; however, this is usuallyimpractical. This technique overcomes this problem by deriving the LUTfrom a relatively small number of patches represented by colors widelyseparated in the color space and obtaining additional patches forselected in-gamut colors as required to improve the LUT accuracy inthose regions of color space where transform accuracy is more important.As mentioned above, one important region contains the points of neutralcolor.

For a LUT used for tetrahedral interpolation, an additional patch for aselected in-gamut color may be obtained by identifying an interior pointof a tetrahedron having four vertices defined by points in the LUT. Aninterior point (x_(P),y_(P),z_(P)) to a specific tetrahedron defined byvertices at points (x₁,y₁,z₁) through (x₄,y₄,z₄) may be determined fromthe expressions:

x _(P) =a ₁ x ₁ +a ₂ x ₂ +a ₃ x ₃ +a ₄ x ₄

y _(P) =a ₁ y ₁ +a ₂ y ₂ +a ₃ y ₃ +a ₄ y ₄

z _(P) =a ₁ z ₁ +a ₂ z ₂ +a ₃ z ₃ +a ₄ z ₄

where a₁ through a₄ are coefficients of interpolation chosen such thata₁+a₂+a₃+a₄=1 and all the coefficients are greater than zero.

Referring to FIG. 3A, suppose points 101-104 represent vertices atpoints (x₁,y₁,z₁,) through (x₄,y₄,z₄), respectively. By setting thecoefficients of interpolation appropriately, any point internal to thetetrahedron may be chosen. Point 107, for example, may be selected bychoosing the interpolation coefficients appropriately.

The interpolation process used to derive a LUT entry from measuredvalues is essentially the same process as that used to implement thedevice transforms themselves. By obtaining additional color patches inselected regions of color space, the distance between the measuredpoints in those regions is reduced and the accuracy of interpolationbetween these more closely spaced points is enhanced. As a result, theaccuracy of the transform implemented by the LUT is also enhanced. Thisprocess may be repeated until a desired level of transformation accuracyis achieved.

Referring to FIG. 16, this process may be carried out by printing colorpatches that correspond to some set of points separated in color spaceby some suitably large distance (step S261) and deriving from thesepatches a LUT with uniform spacing (step S262). The tetrahedral regionsformed by points in the LUT entries are examined to determine if themapping error is unacceptable large (step S263). For any particulartetrahedron, the mapping error may be determined by selecting aninterior point of the tetrahedron, printing a corresponding patch,measuring the actual color of the patch using a measuring device such asa spectral photometer, and comparing the measured values with the valuesobtained by LUT interpolation. The way in which an interior point may beselected is described above.

If no region has an unacceptably large error, the process terminates(step S264), otherwise a point interior to that tetrahedral region isselected and a corresponding color patch is printed (step S265). Thecolor patch is scanned (step S266) and the information in the LUTentries for the four points defining the tetrahedral region are derivedagain using the new color point (step S267). The color patch and scannedvalues used to determine the mapping error may also be used here. Thisprocess reiterates until no tetrahedral region represented by LUTentries has interpolation errors that are unacceptably large.

Improving Accuracy for Out-of-Gamut Colors

The accuracy of LUT entries for out-of-gamut colors can be improved byobtaining additional patches for selected out-of-gamut colors asrequired to improve the LUT accuracy in those regions of color spacewhere transform accuracy is more important. These patches may beobtained from another source such as a different output device or byoperating a given output device in a different mode, as described above.The coordinates of the point in DICS representing the patch color may beobtained from a measuring device such as a spectral photometer.

Referring to FIG. 17, a method for improving the accuracy ofout-of-gamut colors comprises using current LUT entries to obtain aninitial estimate of the patch color space coordinates (step S271),comparing the estimated coordinates to the actual (measured) coordinatesto obtain the estimation error (step S272), determining if theestimation error is acceptably small (step S273) and, if not, modifyingcurrent LUT entries according to the estimation error (step S274). Theprocess terminates (step S275) when the estimation error is acceptablysmall.

An initial estimate of the coordinates (x_(S),y_(S),z_(S)) in the DICSfor a selected out-of-gamut color is obtained by tetrahedralinterpolation of the four points closest to the scanned point accordingto:

x _(S) =a ₁ x ₁ +a ₂ x ₂ +a ₃ x ₃ +a ₄ x ₄

y _(S) =a ₁ y ₁ +a ₂ y ₂ +a ₃ y ₃ +a ₄ y ₄

z _(S) =a ₁ z ₁ +a ₂ z ₂ +a ₃ z ₃ +a ₄ z ₄

where interpolation coefficients a₁ through a₄ are determined by thelocation of the interpolated point relative to the locations of thesefour closest points.

The estimation errors in the x, y and z dimensions are obtained from theexpressions:

e _(x)=x_(m)−x_(S)

e _(y) =y _(m) −y _(S)

e _(z) =z _(m) −z _(S)

where the coordinates x_(m), y_(m) and z_(m) are known from measurementsof the color patch. Alternatively, other measures of estimation errormay be used such as the square of the differences shown above.

The coordinates of the four closest points are modified to obtain alower estimation error. For example, the x coordinates are modifiedreiteratively according to the following expressions:${x_{1}(i)} = {{x_{1}\left( {x - 1} \right)} - {\eta \frac{\partial e_{x}}{\partial x_{1}}}}$${x_{2}(i)} = {{x_{2}\left( {x - 1} \right)} - {\eta \frac{\partial e_{x}}{\partial x_{2}}}}$${x_{3}(i)} = {{x_{3}\left( {x - 1} \right)} - {\eta \frac{\partial e_{x}}{\partial x_{3}}}}$${x_{4}(i)} = {{x_{4}\left( {x - 1} \right)} - {\eta \frac{\partial e_{x}}{\partial x_{4}}}}$

where i=index of iteration, and

η=convergence coefficient.

The y and z coordinates are modified in a similar manner. After eachiteration, a new interpolation is performed according to

x _(S) =a ₁ x ₁(i)+a ₂ x ₂(i)+a ₃ x ₃(i)+a ₄ x ₄(i)

y _(S) =a ₁ y ₁(i)+a ₂ y ₂(i)+a ₃ y ₃(i)+a ₄ y ₄(i)

z _(S) =a ₁ z ₁(i)+a ₂ z ₂(i)+a ₃ z ₃(i)+a ₄ z ₄(i)

and a new estimation error is obtained as explained above. The iterationcontinues until the estimation error is acceptably small.

By using these two techniques, the interpolation accuracy of a LUT forboth in-gamut and out-of-gamut colors may be improved without increasingthe number of entries in the LUT.

Retaining Useful Information in Self-calibration Techniques

The operational characteristics of many input and output devices changewith time. These changes cause shifts in the location and size of devicegamuts in color space. Similar changes may occur as a result ofoperating an output device in a new mode or using different media. As aresult, it is desirable to modify transformations to account for thesechanges so that the reproduction accuracy of a system can be maintained.The following technique accounts for changes in the gamut of outputdevice 30 by deriving a recalibration transformation.

FIG. 8 is a schematic representation of a shift in the gamut of ahypothetical output device such as that caused by component aging. Gamut170 represents the original characteristics of output device 30 whichwas used originally to calibrate the color image reproduction system.The colors represented by points 171 and 173 are in the original gamutand colors represented by points 185 and 188 are outside the originalgamut. Points 185 and 188 are mapped onto the boundary of the originalgamut at points 176 and 179, respectively.

Gamut 180 represents current characteristics of output device 30 whichis to be used to recalibrate the system. According to this new gamut,colors represented by points 181 and 188 are in the current gamut andcolors represented by points 173 and 185 are outside the current gamut.

One way in which a system may be recalibrated is to merely invoke theprocedures described above, deriving new device maps in a manner similarto that done to derive original maps. This approach is not attractivebecause the derivation of an accurate map for an entire gamut canrequire considerable time and expense and because it discards usefulinformation that may not be available at the time of recalibration. Forexample, patches of a particular color that were used to derive anoriginal device map may not be available at the time of recalibration.

The technique described below overcomes these problems by providing forself-recalibration and by restricting the recalibration to those regionsof color space that are inside the current output device gamut. As aresult, LUT entries for colors such as those represented by points 181and 188 are affected by the recalibration process but LUT entries forcolors such as those represented by point 173 are not affected.

Referring to FIGS. 10 and 18, controlling device 20 selects values inthe output-DDCS to cause output device 30 to generate image 37 (step281) containing patches of colors that are inside the current outputdevice gamut. Input device 10 scans image 37 (step 282) to generatevalues within input-DDCS, which input device map 21 and output devicemap 23 collectively transform into values in the output-DDCS (step 283)according to some transformation such as ƒ_(c) discussed above. In someembodiments, input-device map 21 and output-device map 23 are mergedinto a single LUT to reduce the amount of memory required to store thetransformation. In such embodiments, the recalibration techniquedescribed here is especially attractive. Because the output device gamuthas changed, some of the values obtained from output device map 23 willdiffer significantly from the corresponding values used to generate thecolor patches.

These differences can be expressed as a transform e:CMY_(CUR)→CMY_(ORIG)that maps color coordinates CMY_(CUR) pertaining to the current outputdevice gamut to color coordinates CMY_(ORIG) as they pertain to theoriginal gamut. By deriving the inverse functione⁻¹:CMY_(ORIG)→CMY_(CUR) (step 284), recalibrate map 22 can applyappropriate correction to obtain proper values for output device 30.This inverse function is referred to herein as a recalibrationtransformation.

In one embodiment, the recalibration transformation is implemented as aLUT. The recalibration LUT includes information in each table entryindicating whether the respective point represented by that table entryis inside or outside the current output device gamut. During operationof the color image reproduction system, referring to FIGS. 10 and 19,input device 10 scans an original image (step 291), output device map 23subsequently obtains an interim mapping of pixels in the output-DDCS(step 292) and recalibrate map 22 selectively applies the recalibrationtransformation to these mapped pixels as required. This is accomplishedby identifying the four points in output-DDCS (step 293) that define thesmallest tetrahedron enclosing the color point for a respective pixel,and interrogating the recalibration LUT entries (step 294) for thesefour points to determine if all are within the current output devicegamut. If all points are in the current output device gamut, therecalibration transformation is applied (step 295) by performinginterpolation among those four points in the recalibration LUT. Theresults of this interpolation is used to control output device 30 forthat particular pixel.

If at least one of the four points is not within the current outputdevice gamut, the results obtained from the original output device map23 is projected to the neutral-color line and clipped at the currentgamut boundary (step 296). This point at the boundary is used to controloutput device 30 for the respective pixel.

FIG. 11 illustrates major components of one way for using therecalibration transformation discussed above. Path 71 receivesinformation representing points in color space. In one embodiment, thisinformation includes an indication whether each point is in the currentoutput device gamut. In another embodiment, check gamut 73 determinesthis by comparing the color space coordinates of each point with thelocation of the current gamut boundary.

For either embodiment, check gamut 73 controls switch 72 to connect withpath 74 if a respective point p is not in the current gamut. Project andclip 76 projects a line from point p to the neutral-color line anddetermines the intersection or clip point p′ of this projection with theboundary of the current gamut. Information representing clip point p′ ispassed along path 78.

If point p is in the current gamut, check gamut 73 causes switch 72 toconnect with path 75. Recalibration transformation 77 implements the e⁻¹transformation discussed above and passes the results of thistransformation along path 78.

What is claimed is:
 1. A method for deriving a recalibrationtransformation for a color image reproduction system comprising an inputdevice and an output device, said method comprises: obtaining a mediumconveying a selected color within a current gamut of said output devicerepresented by first values in a first color space, scanning said mediumwith said input device to obtain second values representing saidselected color in a second color space, applying a calibrationtransformation to map said second values to third values in said firstcolor space, deriving an error transformation that maps from said firstvalues to said third values, and deriving said recalibrationtransformation from an inverse of said error transformation that mapsfrom said third values to said first values.
 2. A method for using arecalibration transformation in a color image reproduction systemcomprising an input device and an output device, wherein saidrecalibration transformation accounts for differences between anoriginal gamut of said output device and a current gamut of said outputdevice, said method comprises: scanning a first medium conveying animage having one or more in-gamut colors that are within said currentgamut and one or more out-of-gamut colors that are not within saidcurrent gamut to generate respective first values representing saidcolors in a first color space, applying a calibration transformation tomap said first values to second values representing said colors in asecond color space, applying said recalibration transformation to mapsecond values representing only said in-gamut colors to third values,and generating a second medium conveying a replica of said image,wherein said replica is generated in response to said third values andto second values representing only out-of-gamut colors.
 3. A methodaccording to claim 2 wherein said replica is generated in response tofourth values representing points corresponding to an intersection of aboundary of said current gamut with lines projected from said secondvalues to a neutral-color line in said second color space.
 4. Anapparatus for deriving a recalibration transformation for a color imagereproduction system comprising an input device and an output device,said apparatus comprises components that: scan a medium conveying aselected color within a current gamut of said output device representedby first values in a first color space to obtain second valuesrepresenting said selected color in a second color space, apply acalibration transformation to map said second values to third values insaid first color space, derive an error transformation that maps fromsaid first values to said third values, and derive said recalibrationtransformation from an inverse of said error transformation that mapsfrom said third values to said first values.
 5. An apparatus for using arecalibration transformation in a color image reproduction systemcomprising an input device and an output device, wherein saidrecalibration transformation accounts for differences between anoriginal gamut of said output device and a current gamut of said outputdevice, said apparatus comprises components that: scan a first mediumconveying an image having one or more in-gamut colors that are withinsaid current gamut and one or more out-of-gamut colors that are notwithin said current gamut to generate respective first valuesrepresenting said colors in a first color space, apply a calibrationtransformation to map said first values to second values representingsaid colors in a second color space, apply said recalibrationtransformation to map second values representing only said in-gamutcolors to third values, and generate a second medium conveying a replicaof said image, wherein said replica is generated in response to saidthird values and to second values representing only out-of-gamut colors.6. An apparatus according to claim 5 wherein said replica is generatedin response to fourth values representing points corresponding to anintersection of a boundary of said current gamut with lines projectedfrom said second values to a neutral-color line in said second colorspace.
 7. A medium readable by a machine embodying a program ofinstructions for execution by said machine to perform a method forderiving a recalibration transformation for a color image reproductionsystem comprising an input device and an output device, said methodcomprises: obtaining a medium conveying a selected color within acurrent gamut of said output device represented by first values in afirst color space, scanning said medium with said input device to obtainsecond values representing said selected color in a second color space,applying a calibration transformation to map said second values to thirdvalues in said first color space, deriving an error transformation thatmaps from said first values to said third values, and deriving saidrecalibration transformation from an inverse of said errortransformation that maps from said third values to said first values. 8.A medium readable by a machine embodying a program of instructions forexecution by said machine to perform a method for using a recalibrationtransformation in a color image reproduction system comprising an inputdevice and an output device, wherein said recalibration transformationaccounts for differences between an original gamut of said output deviceand a current gamut of said output device, said method comprises:scanning a first medium conveying an image having one or more in-gamutcolors that are within said current gamut and one or more out-of-gamutcolors that are not within said current gamut to generate respectivefirst values representing said colors in a first color space, applying acalibration transformation to map said first values to second valuesrepresenting said colors in a second color space, applying saidrecalibration transformation to map second values representing only saidin-gamut colors to third values, and generating a second mediumconveying a replica of said image, wherein said replica is generated inresponse to said third values and to second values representing onlyout-of-gamut colors.
 9. A medium according to claim 8 wherein saidreplica is generated in response to fourth values representing pointscorresponding to an intersection of a boundary of said current gamutwith lines projected from said second values to a neutral-color line insaid second color space.
 10. An apparatus for deriving a recalibrationtransformation for a color image reproduction system comprising an inputdevice and an output device, comprising: a medium conveying a selectedcolor within a current gamut of said output device represented by firstvalues in a first color space, said input device for scanning saidmedium to obtain second values representing said selected color in asecond color space, a calibration transform for mapping said secondvalues to third values in said first color space, an error transformthat maps from said first values to said third values, and arecalibration transform that maps from said third values to said firstvalues, said recalibration transform comprising an inverse of said errortransform.
 11. An apparatus for using a recalibration transformation ina color image reproduction system comprising an input device and anoutput device, wherein said recalibration transformation accounts fordifferences between an original gamut of said output device and acurrent gamut of said output device, comprising: a first mediumconveying an image having one or more in-gamut colors that are withinsaid current gamut and one or more out-of-gamut colors that are notwithin said current gamut, said input device for scanning said firstmedium to generate respective first values representing said colors in afirst color space, a calibration transform for mapping said first valuesto second values representing said colors in a second color space, arecalibration transform for mapping second values representing only saidin-gamut colors to third values, and said output device for generating asecond medium conveying a replica of said image, wherein said replica isgenerated in response to said third values and to second valuesrepresenting only out-of-gamut colors.
 12. An apparatus according toclaim 4 comprising a scanner for scanning said medium.
 13. An apparatusaccording to claim 4 comprising a printer for printing colorsrepresented by said first values on said medium.
 14. An apparatusaccording to claim 5 comprising a scanner for scanning said firstmedium.
 15. An apparatus according to claim 5 comprising a printer forprinting said replica of said image.
 16. An apparatus according to claim10 wherein said input device is a scanner.
 17. An apparatus according toclaim 11 wherein said input device comprises a scanner.
 18. An apparatusaccording to claim 11 wherein said output device comprises a printer.19. An apparatus for deriving a recalibration transformation for a colorimage reproduction system comprising an input device and an outputdevice, said apparatus comprising: means for scanning a medium conveyinga selected color within a current gamut of said output devicerepresented by first values in a first color space to obtain secondvalues representing said selected color in a second color space, meansfor applying a calibration transformation to map said second values tothird values in said first color space, means for deriving an errortransformation that maps from said first values to said third values,and means for deriving said recalibration transformation from an inverseof said error transformation that maps from said third values to saidfirst values.
 20. An apparatus for using a recalibration transformationin a color image reproduction system comprising an input device and anoutput device, wherein said recalibration transformation accounts fordifferences between an original gamut of said output device and acurrent gamut of said output device, said apparatus comprising: meansfor scanning a first medium conveying an image having one or morein-gamut colors that are within said current gamut and one or moreout-of-gamut colors that are not within said current gamut to generaterespective first values representing said colors in a first color space,means for applying a calibration transformation to map said first valuesto second values representing said colors in a second color space, meansfor applying said recalibration transformation to map second valuesrepresenting only said in-gamut colors to third values, and means forgenerating a second medium conveying a replica of said image, whereinsaid replica is generated in response to said third values and to secondvalues representing only out-of-gamut colors.